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A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" can be traced back to the Roman term opus caementicium, used to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as cementum, cimentum, cäment, and cement.
Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster).
Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting.
Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminiumsilicates[citation needed]) with lime (calcium oxide).
The most important uses of cement are as a component in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.
Chemistry[edit]
Non-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), hardens by carbonation in the presence of carbon dioxide which is naturally present in the air. First calcium oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure:
CaCO3 → CaO + CO2
The calcium oxide is then spent (slaked) mixing it with water to make slaked lime:
CaO + H2O → Ca(OH)2
Once the excess water is completely evaporated (this process is technically called setting), the carbonation starts:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed to air, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the lime cycle.
Conversely, hydraulic cement hardens by hydration when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being:
Belite (2CaO•SiO2);
Alite (3CaO•SiO2);
Tricalcium aluminate (3CaO•Al2O3) (historically, and still occasionally, called 'celite');
Brownmillerite (4CaO•Al2O3•Fe2O3).
The silicates are responsible of the mechanical properties of the cement, the tricalcium aluminate and the brownmillerite are essential to allow the formation of the liquid phase during the kiln sintering (firing). The chemistry of the above listed reactions is not completely clear and is still the object of research.[
Safety issues[edit]
Bags of cement routinely have health and safety warnings printed on them because not only is cement highly alkaline, but the setting process is exothermic. As a result, wet cement is strongly caustic (water pH = 13.5) and can easily cause severe skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation. Some trace elements, such as chromium, from impurities naturally present in the raw materials used to produce cement may cause allergic dermatitis.[1] Reducing agents such as ferrous sulfate (FeSO4) are often added to cement to convert the carcinogenic hexavalent chromate (CrO42-) in trivalent chromium (Cr3+), a less toxic chemical species. Cement users need also to wear appropriate gloves and protective clothing
Setting and curing[edit]
Cement starts to set when mixed with water which causes a series of hydration chemical reactions. The constituents slowly hydrate and the mineral hydrates solidify; the interlocking of the hydrates gives cement its strength. Contrary to popular perceptions, hydraulic cements do not set by drying out; proper curing requires maintaining the appropriate moisture content during the curing process. If hydraulic cements dry out during curing, the resulting product can be significantly weakened.
Environmental impacts[edit]
Cement manufacture causes environmental impacts at all stages of the process. These include emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries, and damage to countryside from quarrying. Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them.
Green cement[edit]
Green cement is a cementitious material that meets or exceeds the functional performance capabilities of ordinary Portland cement by incorporating and optimizing recycled materials, thereby reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable construction material.
The manufacturing process for green cement succeeds in reducing, and even eliminating, the production and release of damaging pollutants and greenhouse gasses, particularly CO2.
Growing environmental concerns and increasing cost of fuels of fossil origin have resulted in many countries in sharp reduction of the resources needed to produce cement and effluents (dust and exhaust gases).[55]
Peter Trimble, a design student at the University of Edinburgh has proposed 'DUPE' based on Sporosarcinapasteurii, a bacterium with binding qualities which, when mixed withsand and urine produces a concrete said to be 70% as strong as conventional material
ENVIRONMENTAL IMPLICATIONS
Many of the aspects of the cement making process are potentially environmentally damaging,
although these risks can be minimised. The areas of potential concern are listed below.
Dust emissions
The manufacture of cement generates large quantities of dust. These must be prevented (both
on environmental and economic grounds) from escaping to the atmosphere. The two areas
where dust has the potential to escape are via air streams that have been used to carry cement
(e.g. the mills or kiln) and directly from equipment used to transport cement (e.g. the various
conveyor belts). Thus to prevent dust emissions all transport equipment is enclosed, and the
air both from these enclosures and from the kiln and mills is treated in an electrostatic
precipitator to remove its load of dust. Here dust-laden air passes between an electrode
carrying 50 000 volts and an earthed collection plate. The electrostatic discharge between the
electrode and the plate forces the dust onto the plates, from which it is removed.
The current emission limit from the main stack at Golden Bay is 250 mg m-3 and at Milburn
is 150 mg m-3, while in Europe emission limits of down to 50 mg m-3 are becoming common.
This poses a significant challenge to the manufacturing operation both in capital cost to
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reduce emissions and monitoring of emissions to ensure compliance with existing resource
consents.